Ion mobility spectrometers (IMS) can identify material from a sample of interest by ionising the material (e.g., molecules, atoms, and so forth) and measuring the time it takes the resulting ions to travel a known distance under a known electric field. Each ion's time of flight is associated with the ion's mobility. An ion's mobility relates to its mass and geometry. Therefore, by measuring the time of flight of an ion it is possible to infer its identity. These times of flight may be displayed graphically or numerically as a plasmagram.
Some IMS cells include detectors which collect ions to measure their time of flight so they can be identified, this may be done in the presence of a drift gas so that mobility effects can separate the ions. Some IMS cells may separate ions according to their time of flight so that ions having selected times of flight (implying a selected range of ion mobilities) can be provided to other detector instruments, such as mass spectrometers, for further analysis. One example of this technique is known as IMS-MS, in which an IMS cell is used as an ion filter to select ions from a sample. The selected ions are then provided to a mass spectrometer. In such ion identification or filtering methods, groups of ions can be released from a reaction region by opening an ion shutter and/or passed into an inlet of a mass spectrometer.
The reaction region of an IMS cell has a finite length, and in the time interval for which the shutter is held open, ions which may be distributed around the reaction region must travel (at least partially) across that reaction region to reach the shutter. The inventor in the present case has appreciated that this means that holding the shutter open only for a short interval reduces the number of slow moving ions that are able to travel through it in that interval. The inventor in the present case has recognised that this may reduce sensitivity of the detector to slow moving ions. After ions have passed through the shutter, their motion along the drift chamber is dependent upon the voltage profile in that drift chamber. He has further appreciated that the action of closing the shutter may modify the profile voltage near to the shutter. If so, the back of a group of ions in the drift chamber may experience a profile voltage different from that which the front of that group did when at that same position. The inventor concludes from this that this can retard or accelerate the ions at the back of the group relative to the rest of the group.
This leads him to believe that these factors may adversely affect the resolution and sensitivity of an IMS cell.